1. Field of the Invention
Embodiments of the present invention are directed to phosphor compositions that comprise a BAM phosphor BaMgAl10O17:Eu2+ and at least one other hexaluminate that may also be europium activated.
2. Description of the Related Art
Owing to its high light output and excellent display of color (as represented, for example, on a CIE color diagram), the known BAM phosphor BaMgAl10O17 doped with divalent europium (Eu2+) has been widely used as the blue phosphor component in applications such as fluorescent lamps, light emitting diode (LED) and plasma display panels (PDP). In plasma display panel applications, BAM:Eu is conventionally adopted as the blue-emitting component under vacuum ultraviolet (VUV) excitation; however, considerable degradation in light output and color shift (toward the green) are known to be problematic. This is thought to be due to an annealing procedure during the manufacturing process, as well as to plasma radiation and/or sputtering damage that occurs during day-to-day use.
BAM has a β-alumina structure, and belongs to the family of hexaluminates. Hexaluminates have a column-like structure and consists of blocks of cubic closed packed (CCP) oxygen layers with cations in tetrahedral and octahedral interstices. Because the blocks are quite similar to the MgAl2O4 spinel structure, hexaluminates are often referred to as “spinel blocks.” The blocks are separated by mirror planes, which contain the large cations.
The magnetoplumbite structure is similar to the β-alumina structure as both have identical spinel blocks. The difference between them lies in the mirror planes, which are loosely-packed in the magnetoplumbite case, and tightly-packed in the β-alumina structure. LaMgAl11O19 is a typical magnetoplumbite. Structurally, its formula can be rewritten as [LaAlO3][(Al3Mg)A7O16], where [LaAlO3] are the ions on the mirror plane, and the [(Al3Mg)Al7O16] portion of the structure exists as the above-mentioned spinel blocks. In the formula (Al3Mg) are groups of ions having a 4-fold coordination, and the aluminum as part of the Al7 oxide has a six-fold coordination. In the same manner, β-alumina BaMgAl10O17 can be rewritten as [BaO][(Al3Mg)A7O16], where just two ions Ba and O are presented on the mirror plane.
Diagrams of the atomic arrangements (and hence crystal structure) of three hexaluminates are shown in FIGS. 1A-1C. FIG. 1A is β-alumina as represented by the formula NaAl11O17, which may be re-written as [NaO][Al4Al7O16]; FIG. 1B is the BAM compound BaMgAl10O17, which may be written as [BaO][(Al3Mg)Al7O16]; and FIG. 1C is the magnetoplumbite LaMgAl11O19, which may be written as [LaAlO3][(Al3Mg)Al7O16].
What is needed in the art are phosphor compositions that enhance the emission intensity and degradation resistance of the conventional BAM phosphor (BaMgAl10O17:Eu2+), utilizing properties afforded by mixing the conventional BAM phosphor with other hexaluminates.